[FEATURE] Support application transparency

[xxxserxxx]

According to tcell documentation, it should be able to create transparent UIs by using tcell.SetBackgroundColor(). I've so far been unsuccessful by following the examples, but haven't spent a lot of time on it.

This would be a nice-to-have option for users via a "-transparent" flag.

[xxxserxxx]

@spezifisch, can we take this out for milestone 1.0? It looks like there are unresolved issues in tview that might make this difficult, and I think it'd be nice to reach a 1.0 release. I know I created the ticket, but I'd like to kick it down the road; I think it'd touch a lot of files, and it might ultimately get blocked by issues in tview.